
Did you have blood tests confirming ovulation before the matings, if not then she may not have ovulated till after the matings and then 48hrs for eggs to ripen during which time the sperm has remained viable.
My own bitch had her first litter on day 61 from one mating on day 12 and her second litter on day 63 of the second matings on day 12 and 14.
So no two gestational periods are the same in any one bitch.
I hope all goes well, get her checked over at the vets as primary inertia caused by a big litter or singleton can prevent labour ever starting naturally.